1. Field of the Invention
This invention relates to hydrographic surveys and is directed more particularly to a method for determining on a continuous basis, the height of water above a chart datum as a survey vessel performs hydrographic surveys in a waterway, to provide more accurate depth data.
2. Description of the Prior Art
The traditional method of coastal hydrographic surveying relies on a tide gauge, i.e., a water level gauge. The tide gauge records water heights at a selected specified time interval, usually 6 or 10 minutes. A tide staff and a human recorder may also be used to record the tide at the selected specified time interval. The time indexed tide is subtracted from time indexed soundings to provide data for bathymetric maps, or nautical charts.
Tide (coastal projects) and stage (river projects) are vertical measurements relative to a specific water level in a navigation channel. Typically, the vertical reference is a low water event in order to maintain the minimum depth capable of transporting a vessel safely to port. The vertical measurements are recorded by a tide gauge at a specific period of time. Since the number of gauges are limited, water levels are modeled from the gauge, up and downstream, at a distance from the gauge. These modeled water levels, called "Tidal Zoning" are used to reduce soundings measured in a channel to a common reference, or chart datum. The problem with this approach is the use of averaged parameters. The parameters are "time" and "height" change at a distance from the gauge. Wind and changes in atmospheric pressure disrupt the average ambient conditions which, in turn, cause the models to provide false water level relationships during a hydrographic survey. Hydrographic surveys relative to a tide gauge develop ever increasing errors as the survey vessel travels away from the tide gauge. The disparities are of greater magnitude in areas that have a large range of tide.
Vessels maneuvering through a waterway typically use acoustic transducer depth-determining devices in an effort to ascertain hull clearance above a channel bottom. Unfortunately, the configuration and depth of the channel bottom varies because of shoaling. To compensate for vagaries in channel bottom depths, vessels frequently are loaded short of a full load to allow for a higher vessel bottom position above the channel bottom.
Accordingly, there is a need for improved vertical position data for vessels traversing waterways, so as to facilitate more accurate hydrographic surveys, which will produce more accurate indications onboard waterborne vessels as to the clearance beneath the keel of the vessel to the published channel depth.